1. Field of the Invention
This invention relates in general to a direct access storage device (DASD) of the type utilizing magnetoresistive read sensors for reading signals recorded in a magnetic medium and, more particularly, to a DASD having a novel multi-state preamplifier for increasing the high frequency bandwidth of the read signal channel.
2. Description of Related Art
Moving magnetic storage devices, especially magnetic disk drives, are the memory devices of choice. This is due to their expanded non-volatile memory storage capability combined with a relatively low cost.
Magnetic disk drives are information storage devices which utilize at least one rotatable magnetic media disk having concentric data tracks defined for storing data, a magnetic recording head comprising a read sensor and a write transducer for reading data from and/or writing data to the various data tracks, a slider for supporting the transducer in proximity to the data tracks typically in a flying mode above the storage media, a suspension assembly for resiliently supporting the slider and the transducer over the data tracks, and a positioning actuator coupled to the transducer/slider/suspension combination for moving the transducer across the media to the desired data track and maintaining the transducer over the data track center line during a read or a write operation. The transducer is attached to or is formed integrally with the slider which supports the transducer above the data surface of the storage disk by a cushion of air, referred to as an air-bearing, generated by the rotating disk.
Alternatively, the transducer may operate in contact with the surface of the disk. Thus, the suspension provides desired slider loading and dimensional stability between the slider and an actuator arm which couples the transducer/slider/suspension assembly to the actuator. The actuator positions the transducer over the correct track according to the data desired on a read operation or to the correct track for placement of the data during a write operation. The actuator is controlled to position the transducer over the desired data track by shifting the combination assembly across the surface of the disk in a direction generally transverse to the data tracks. The actuator may include a single arm extending from a pivot point, or alternatively a plurality of arms arranged in a comb-like fashion extending from a pivot point. A rotary voice coil motor (vcm) is attached to the rear portion of the actuator arm or arms to power movement of the actuator over the disks.
The vcm located at the rear portion of the actuator arm is comprised of a top plate spaced above a bottom plate with a magnet or pair of magnets therebetween. The vcm further includes an electrically conductive coil disposed within the rearward extension of the actuator arm and between the top and bottom plates, while overlying the magnet in a plane parallel to the magnet. In operation, current passes through the coil and interacts with the magnetic field of the magnet so as to rotate the actuator arm around its pivot and thus positioning the transducer as desired.
The magnetic media disk or disks in the disk drive are mounted to a spindle. The spindle is attached to a spindle motor which rotates the spindle and the disks to provide read/write access to the various portions on the concentric tracks on the disks.
One or more electrical conductors extend over the suspension assembly to electrically connect the read/write transducer to a read/write chip on the actuator arm. A multiline flexible printed circuit cable (actuator flex cable) provides the electrical contact between the read/write chip and the disk drive electronics which are mounted outside the disk drive housing. Inside the disk drive housing, the actuator flex cable connects to an electrical connector pin assembly, which in turn, through an opening or connector port in the housing, connects to the external electronics.
In high capacity disk drives, magnetoresistive read sensors or giant magnetoresistive read sensors, commonly referred to as MR heads and GMR heads, respectively, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The changes in resistance of the MR element in response to magnetic data recorded on a disk surface are amplified in the read/write chip (also referred to as the arm electronics (AE) module) on the actuator arm before transmission to the external electronics. The frequency response of the preamplifier in the AE module, and in particular its high frequency bandwidth determines the data rate capability of the disk drive. The high frequency bandwidth of the system comprising the MR element, preamplifier and interconnects is a function of the MR element resistance. MR element resistances generally have a range of values due to manufacturing variations and tolerances. The resistance of a single MR element may also change due to temperature or other conditions in the disk drive during manufacturing and use.
In order to optimize the MR element performance, one approach of the prior art has been to assume a resistance value based on statistical data and to use this value to optimize the operating point of the MR element for optimum performance. Christner et al. in U.S. Pat. No. 5,412,518 disclose a method for controlling the biasing current applied to the MR elements within a disk drive to provide optimized bias current for each head/disk/channel component combination based on measurements of the resistance of each MR element in the disk drive prior to assembly. Contreras et al. in U.S. Pat. No. 5,774,291 disclose a circuit for measuring the MR element resistance after assembly into the disk file for optimizing the MR element's operating point. Klein et al. in U.S. Pat. No. 5,122,915 disclose a preamplifier for an MR sensor that maintains a constant voltage across the MR element and provides a voltage signal output. However gain-bandwidth product limitations exist that are believed due to parasitic capacitances. Klein et al. in U.S. Pat. No. 5,444,579 disclose a preamplifier that uses a current mode amplifier resulting in increased gain-bandwidth product. However, the prior art does not address optimization of the preamplifier bandwidth as a function of MR element resistance.
Therefore, there is a need for an preamplifier that improves the bandwidth of the MR element/preamplifier/inteconnect system and, in particular, optimizes the high frequency bandwidth of the system for a range of resistances of the MR elements.